Electrically-active implantable bio-medical devices (such as for example pacemakers, cochlear implants, and neural prosthetics) are increasing in popularity due to the potential of continuous monitoring, instantaneous and directed delivery of treatments, reduction of treatment costs, and unique treatment options. However, because many of the component materials used in such devices are not bio-compatible, that is, they are toxic to the body and can induce undesirable biological reactions, it is critical to hermetically seal the non-bio-compatible components (e.g. CMOS, passive components, batteries) in a bio-compatible material, so that the body does not have a cyto-toxic response. Hermetic sealing also helps protects electrical components from damage due to moisture and the corrosive environment in the body. FIG. 1 shows a schematic illustration of a common hermetic encapsulation approach for implantable devices, such as 10, where non-bio-compatible components and materials 11, such as electronics, are encapsulated in a hermetically sealed package 12 made of bio-compatible materials. In this arrangement, an array of hermetic electrically conducting feedthroughs 13 is provided on an electrically insulating portion 14 of the package 12 for use as electrical conduits which allow communication of electrical signals between the body and electronics within the package.
Various methods are known to produce hermetic electrically conducting feedthroughs. However, they often tend to be high-cost, lack scalability, and have inherent material incompatibilities. For example, FIGS. 2A and 2B illustrate a method for producing metal feedthroughs in laser drilled holes on non-conductive substrates. In this method, a ceramic or other electrically non-conductive substrate 20 is laser drilled with holes 21. The holes are turned into feedthroughs by filling them with thick-film metal paste 22 which consists of metal particles in an organic solvent. The metal paste is typically pulled through the holes using vacuum and fired at high temperature to drive out the solvent, leaving only metal in the holes. This method however can be problematic because the thick-film metal paste can leave voids when fired or the adhesion of metal to substrate may be poor, either of which can cause leakage paths through the feedthroughs leading to hermetic failure. Also the high-temperature firing can cause delamination of the metal from the ceramic due to the stresses induced from thermal expansion mismatch between the metal and the ceramic. And because hermetic package enclosures are typically made of bio-compatible metals which must be hermetically bonded to ceramic feedthrough substrates using a high temperature brazing process, this introduces an additional high temperature process which can further increase the chances of failure at the feedthrough-ceramic and also the ceramic-package interfaces. And the laser cutting process used to form holes can introduce additional limitations. For example, laser cutting can cause micro-cracks in the ceramic substrate, making it fragile and limiting the minimum gap between adjacent holes. And the minimum diameter of the substrate holes is restricted due to tapering produced by the laser cutting process which limits feedthrough density. As illustrated in FIGS. 3A and 3B, there exists a trade-off between substrate thickness (scalability) and hermeticity. Shorter holes (in which shorter feedthroughs 26 are formed) in thinner ceramic substrate 25 of FIG. 3B, are easier to laser cut, but they are less likely to be hermetic since there is a smaller area for the metal to adhere to the ceramic. Thicker ceramic substrates, such as 23 in FIG. 3A, provide more surface area for the metal to adhere and improve hermeticity. However, they are harder to laser cut, And as can be seen by the four longer feedthroughs 24 in FIG. 3A in the same substrate area as six shorter feedthroughs 26 in FIG. 3B, feedthrough density is less than a thinner substrate due to hole taper.
Another known method of producing hermetic electrically conducting feedthroughs uses co-fired multi-layer ceramics, and illustrated in FIGS. 4A-4D. In this method, multiple layers of thin ceramics 30-33 are physically punched with holes 34-37, respectively. Each ceramic layer 30-33 is then metalized using thick-film metal paste, 38-41, respectively, to create the feedthroughs in the holes. As shown in FIG. 4D, the layers of ceramics 30-33 are then stacked and co-fired to create the final substrate with feedthroughs 43 extending through the stack. However, the size of holes formed using this method is often restricted to the dimension of the punching process (e.g. about 100-125 microns). And the mechanical fragility of substrates due to punching can restrict the pitch between adjacent holes.
In order to improve the longevity and effectiveness of implantable devices, it is advantageous to be able to fabricate durable hermetic electrically conductive feedthroughs which allow connection to hermetically sealed electronic devices. In particular, it would be advantageous to provide a scalable fabrication method for producing high-density, bio-compatible, hermetic electrically conductive feedthroughs in a range of substrate thicknesses, that improves the hermetic bond between feedthrough and insulator by using lower temperature process for insulator sealing